The present invention broadly relates to methods and apparatus for inspecting surface features of objects, and deals more particularly with a system for detecting surface defects such as holes and bumps in the surface of a semiconductor wafer.
In the semiconductor manufacturing industry, maintaining and improving yield is an increasingly greater challenge as circuit designs become increasingly smaller. Foreign particles and process defects can seriously limit yields during the manufacturing of semiconductor wafers, consequently a great deal of resources has been directed toward developing sophisticated inspection systems for detecting particles and surface defects at the earliest possible stage during wafer fabrication. When the inspection process indicates a number of defects, the wafer may be sent back for re-cleaning. If the defects are particles or other debris on the wafer surface, the re-cleaning process is often successful. However if the defects consist of holes or bumps inherent in the wafer surface they may not be removed by re-cleaning. Because such surface inspection systems fail to distinguish between pit defects and particle defects the wafer is typically sent back for re-cleaning regardless of whether the defects are holes or particles.
Defects in the form of structural flaws, process residues and external contamination occur during the production of semiconductor wafers on a fairly routine basis. The defects are typically detected by a class of instruments referred to as defect scanners. These instruments automatically scan wafer surfaces and detect optical anomalies using a variety of techniques. The location of these anomalies with respect to the pattern of semiconductor devices on the wafer surface is recorded, and this information is often stored and accumulated to form a defect map. This map enables a human operator to systematically inspect each defect under a microscope so that the defect can be characterized according to type (particle, hole, scratch or contaminate). Information gained from this inspection process is then used to correct the source of the defects, and thereby improve efficiency and yield of the semiconductor commercial process.
The inspection process performed by a human operator utilizes a conventional optical microscope, or a much more complicated scanning electron microscope. The wafer is illuminated using any of a variety of techniques, including brightfield illumination, darkfield illumination or spatial illumination filtering. Brightfield illumination of the wafer involves lighting the specimen with a solid cone of rays. The transmitted brightfield illumination is normally performed by a sub-stage condenser and the reflected brightlight illumination is performed by a vertical illuminator. In brightfield illumination, the specimen appears dark against a light background. Brightfield imaging tends to scatter small particles away from the collecting aperture which results in reduced returned energy. When a particle is small compared to the optical point spread function of the lens and is small compared to the digitizing pixels, the brightfield energy from the immediate area surrounding the particle typically contributes a large amount of energy. The small reduction in return energy resulting from the small particles makes the particle difficult to detect. Further, the reduced level of return energy from small particles is often masked out by reflectivity variations from the bright surrounding background such that small particles cannot be detected without numerous false detections. Additionally, if the small particle is on an area of low reflectivity, which may occur for some process layers on wafers, the resultant background return is low and any further reduction due to the presence of a particle becomes thus very difficult to detect.
Brightfield microscopy apparatus relies upon light from a lamp source being gathered by the sub-stage condenser and shaped into a cone whose apex is focused at the plane of the specimen (wafer surface). Specimens are seen because of their ability to change the speed and path of the light passing through them. This ability is dependent on upon the refraction index and opacity of the specimen. To see a specimen in a brightfield microscope, the light rays passing through it must be changed sufficiently to be able to interfere with each other, which produces contrast (differences in light intensities) and thereby builds an image. If the specimen has a refraction index similar to the surrounding medium between the microscope stage in the objective lens, it will not be seen. If the wafer surface defect and the area surrounding the defect do not possess the proper refractive indices, the defect will not have sufficient contrast to be seen.
Darkfield microscopy relies on a different illumination system. Rather than illuminating the sample with a solid cone of light, the condenser is designed to form a hollow cone of light. The light at the apex of the cone is focused at the plane of the specimen; as this light moves past the specimen plane it spreads again into it a hollow cone. The objective lens sits in a dark hollow of this cone (although the light travels around and past the objective lens, no rays enter it). The entire field appears dark when there is no sample on the microscope stage, thus the name xe2x80x9cdarkfield microscopyxe2x80x9d. When a sample is on the stage, the light at the apex of the cone strikes it. The image is made only by those rays scattered by the sample and captured in the objective lens. The image appears bright against a dark background. The advantage of the darkfield imaging is that flat specular areas scatter very little light back toward the detector, resulting in a dark image. Darkfield illumination provides a larger pixel to defect ratio, permitting faster inspections for a given defect size and pixel weight. Darkfield imaging also permits the Fourier filtering to enhance signal to noise ratios.
Any surface features or objects protruding above the surface of the wafer scatter more light toward the detector in darkfield imaging. Darkfield imaging thus produces a dark damage except where circuit features or particles or other irregularities exist.
From the above, it is apparent that brightfield and darkfield microscopy each have their own advantages in detecting certain surface defects, but neither is effective in detecting the full range of defects that can occur on the surface of a semiconductor wafer. In the past, it has been particularly difficult to detect both xe2x80x9cbumpsxe2x80x9d and holesxe2x80x9d in the surface of a wafer using only one type of microscopy, i.e. darkfield or brightfield. It would therefore be desirable to provide an improved optical inspection system that utilizes an illumination system capable of revealing both bumps and holes in a wafer surface. The present invention is directed toward satisfying this need.
According to one aspect of the invention, an optical inspection system for detecting defects in the surface of a semiconductor wafer is provided comprising a light source, an optical assembly for illuminating the wafer, and an objective for collecting light from the wafer and forming a resultant image. The optical assembly includes components that produce simultaneous darkfield and brightfield illumination of the wafer surface. The objective collects both light scattered from the wafer as a result of the darkfield illumination, and light which passes directly through the wafer as a result of the brightfield illumination. The optical assembly includes a light stop having a central circular opening for allowing a beam of light to pass therethrough which is used to provide brightfield illumination, and a ring-shaped opening concentrically disposed around a central opening. The ring-shaped opening allows a hollow cylinder of light to pass through the stop, which is used to produce darkfield illumination of the wafer. A condenser focuses both the hollow cone of light and the light beam substantially at the plane of the wafer surface. Light rays from the beam are refracted through the wafer and pass then directly into an objective lens. Scattered light from the wafer surface produced by the darkfield illumination is also received by the objective lens which forms an image of the wafer surface produced by both types of illumination. In a preferred embodiment, the illumination system is modulated by varying the size of the brightfield light beam and/or by tilting the illumination system relative to the wafer surface in order to improve contrast and the likelihood of detecting surface defects.
According to another aspect of the invention, a method is provided for detecting defects on the surface of a semiconductor wafer, comprising the steps of producing mixedfield illumination of the image of the wafer surface by simultaneously forming darkfield and brightfield illumination of the wafer surface, and viewing the mixedfield image. A darkfield image is formed by illuminating the wafer surface with a hollow cone of light and then collecting light scattered from the surface of the wafer. The brightfield image is formed by focusing solid cone or beam of light on the wafer surface and then collecting light emanating from the surface which originates from the beam. The hollow cone of light and the solid beam are preferably formed by passing light from a source thereof through openings in a single, opaque light stop. The method further includes the step of tilting the hollow cone of light and the solid light beam relative to the wafer surface, and modulating the light beam by changing the size of the beam.
Accordingly, it is the primary object of the present invention to provide an optical inspection system for detecting a wide range of defects in the surface of a semiconductor wafer, including bumps and holes or pits.
Another object of the invention is to provide a system as described above which simultaneously utilizes both brightfield and darkfield illumination of the wafer being inspected.
A further object of the invention is to provide a system as described above which possesses a minimum number of components, including a single light stop and a single source of illumination.
A still further object of the invention is to provide an inspection system of the type mentioned above which allows for modulation of the illumination system so as to maximize the probability of detecting surface defects.
These, and further objects and advantages of the present invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the present invention.